Some of conventional battery modules include plurality of battery cells arranged in parallel and accommodated in a casing (e.g., see JP 2003-346749 A).
The casing of the conventional battery module is made of synthetic resin material. Therefore, since the casing does not have sufficient strength, when mounted in a vehicle, the battery module is possibly damaged due to vibration. An attempt to enhance the strength can causes another problem where a volume occupied by the casing is increased, which results in that downsizing requirement cannot be satisfied.
Therefore, metal material is used for the casing in order to enhance its strength. Usually, a surface of such a metal casing is subject to coating for rust prevention.
In the case of the battery module adopting the metal casing, if battery cells and the casing come into contact with each other due to such reason as battery cells are obliquely disposed in the casing at the time of manufacturing and the battery cells and the casing come into contact with each other, then slight current flow can be caused. This slight current flow can reduce battery voltage. Therefore, it is necessary to inspect whether insulation of the casing is maintained.
However, since the surface of the metal casing is subject to coating, it is difficult to inspect the insulation.